Abstract The goal of the proposed studies is to understand the signaling transduction mechanism governing angiogenesis, an important process in growth and development of tissues, as well as in wound healing processes. It also occurs in diseases, such as cancer, diabetic blindness, and rheumatoid arthritis. In this project, we focus on the roles and mechanisms of tensin signaling in endothelial cells during angiogenesis. The tensin family plays critical roles in organizing the subcellular structure and mediating signaling transductions at focal adhesions, which are the transmembrane structures linking the extracellular matrix to the cytoskeleton. The four members of tensin (tensin1, tensin2, tensin3, and cten) bind to the cytoplasmic tails of ? integrin through their PTB (phosphotyrosine-binding) domains and interact with actin filaments (except cten) via their N-terminal regions, allowing tensins to bridge the actin cytoskeleton to integrin receptors. In addition, tensins contain an SH2 (Src homology 2) domain that interacts with tyrosine-phosphorylated as well as non- phosphorylated proteins and form signaling complexes at focal adhesions. Our recent studies using knockout mice showed that lack of tensin1 impairs tube formation activities in endothelial cells and angiogenic processes in mice, indicating critical involvements of tensins in angiogenesis. However, not all tensins play similar roles in cellular activities. We found that tensin1 and tensin2 promote endothelial cell migration, a critical step during angiogenesis, whereas tensin3 suppresses it. Why highly homologous tensins exert opposite biological activities? By using fluorescent-tagged tensins and live-cell confocal microscopy, we observed that tensins show different spatiotemporal localization patterns in migrating cells. These findings lead us to investigate the roles and regulatory mechanisms of tensins in angiogenesis. We hypothesize that tensins regulate angiogenesis through their common and unique roles, which are dictated by their spatiotemporal localizations and associated molecules, in endothelial cell adhesion, migration, and vascular lumen formation. Three specific aims are proposed to (Aim 1) determine the primary control of spatiotemporal localizations of tensins during in vitro tube formation; (Aim 2) establish the roles and mechanisms of tensins in regulating endothelial cell tube formation; and (Aim 3) investigate the functions and regulatory mechanisms of tensins in angiogenesis using knockout mice. Our research design is innovative because it probes novel and distinct functions of tensins in endothelial cells, and employs a multidisciplinary approach that integrates biochemistry, cell and molecular biology, live-cell fluorescence microscopy, cell culture and mouse models to understand the roles of tensins in angiogenesis. This project has very high clinical and translational relevance that may offer new insights for therapeutic applications to angiogenic related diseases.